1. Field of the Invention
The present invention relates to nuclear magnetic resonance tomography devices for visual representation of regions of the interior of an examination subject, and to methods for operating such devices.
2. Description of the Prior Art
Nuclear magnetic resonance tomography devices are known in the art wherein a patient is disposed within a fundamental magnetic field and a plurality of gradient magnetic fields and radio-frequency excitation pulses are applied to the examination subject to displace the axes of nuclear spin in the examination subject from an equilibrium position so that nuclear magnetic resonance signals generated by the spin precession during relaxation can be obtained.
Such an apparatus is generally described, for example, in German OS No. 33 40 523, corresponding to copending U.S. Application Ser. No. 666,656, filed Oct. 31, 1984 (Wilfried Loeffler and Arnulof Oppelt). The conventional elements of such an apparatus are schematically shown in FIG. 1.
The apparatus shown in FIG. 1 includes coils 1, 2, 3 and 4 connected to a power supply 11 for generating a constant fundamental magnetic field B.sub.0. An examination subject 5, such as a patient, is disposed within the coils 1 through 4. A plurality of sets of gradient coils are also provided, in which the examination subject 5 is disposed, the gradient coils being arranged for generating orthogonal magnetic field gradients in the x, y and z directions in accordance with the coordinate system 6 shown in FIG. 1. For simplicity, only gradient coils 7 and 8 are shown in FIG. 1, connected to a power supply 12. The coils 7 and 8 generate an x-gradient in combination with a pair (not shown) of identical gradient coils disposed on the opposite side of the subject 5. Coils (not shown) for generating a y-gradient are disposed above and below the examination subject 5, parallel thereto. Another set of gradient coils (not shown) for generating the z-gradient field are disposed at the head and feet of the examination subject 5 at a right angle relative to the longitudinal axis of the subject 5.
A radio-frequency coil 9 is provided in this known arrangement which simultaneously functions to excite nuclear resonance in the subject 5, and to acquire the resulting nuclear magnetic resonance signals.
The elements 1 through 9 bounded by the dot-dash line 10 constitute the actual examination apparatus in which the patient is disposed. The power supplies 11 and 12, and other elements described below, are connected thereto. The radio-frequency coil 9 is supplied with radio-frequency pulses from a radio-frequency oscillator 15 through a modulator 13, such as a switch, and a transmission amplifier 14. Nuclear magnetic resonance signals are acquired by the coil 9 and supplied through a signal amplifier 16 and a phase-sensitive recitifer 17 to a process control computer 18. The computer 18 constructs an image from the acquired data, which is visually displayed on a monitor 19. The computer 18 also controls operation of the modulator 13 via a control line 20, and controls operation of the gradient fields power supply 12 via a control line 22. The output of the oscillator 15 is also supplied to the phase-sensitive rectifier 17 via a line 21, and is thus also supplied to the computer 18.
In order to produce an image of the examination subject 5 by means of a nuclear magnetic resonance, for example, the radio-frequency field may be energized in the presence of an z-gradient field. The resonance condition for only one slice in the examination subject 5 will thus be satisfied, so that a selective excitation of the nuclear spins occurs only in this slice. By means of subsequently generating x- and y-gradients, an image can be obtained for the nuclear spin distribution of the examination subject 5. This can be undertaken based on known methods such as the Fourier transformation method or the projection reconstruction method. Both methods are described, for example, in European Application No. 00 74 022.
A problem in the operation of a nuclear magnetic resonance tomography apparatus of the type described above is that the radio-frequency coil must be exactly balanced.